Device and use of the device for measuring the density and/or the electron temperature and/or the collision frequency of a plasma.
Plasmas—electrically activated gases—are used in various technical areas, wherein the particular physical properties of plasmas frequently form the basis for innovative products and processes. Essential for the success of a method based on the use of technological plasmas is the accurate monitoring and—in case of deviations—eventual readjustment of the plasma state. An important characteristic quantity of plasmas is the location-dependent and time-dependent electron density ne, which must be known in order to assess the properties of plasmas. The electron temperature Te and the collision frequency v also play an important role in the assessment of a plasma. The electron temperature is a measure of the activity of a plasma, the collision frequency provides information about the neutral gas composition and the neutral gas temperature, which are important, for example, for the endpoint detection in etching processes. With technologically used plasmas, the determination of the electron density is especially difficult in the so-called reactive plasmas. Only few processes are compatible with industrial processes, i.e. robust enough against pollution and disturbances without affecting the process to be monitored, with simultaneously low expenditure in the measurement process, in the analysis and with respect to online capability
A method suitable for the industrial plasma diagnostics is the plasma resonance spectroscopy. In this method, a high-frequency signal in the gigahertz range is injected into the plasma. The signal reflection is measured as a function of the frequency. Specifically, the resonances are measured as maxima in the absorption. The position of these maxima is a function of the desired central plasma parameter, the electron density, which can at least in principle be determined in this way absolute and without calibration. The shape of the impulse response and the damping of the maxima, respectively, is a function of the electron temperature and the collision frequency, thus allowing conclusions to be drawn about the other characteristic quantities of the plasma. Compared to standard plasma diagnostics, high-frequency measurements have little to no effect on the technical process and are largely insensitive to contamination. Therefore, little investment and maintenance are required, so that the plasma resonance spectroscopy is distinguished by an easy system integration as well as the speed of the measurement process and its fundamental online capability.
A disadvantage of the plasma resonance spectroscopy is that the evaluation of the measurement results, i.e. for example to the electron density inferred from the resonance curve, requires a mathematical model. The spatial resolution of the measurement results, i.e. the determination of the characteristic plasma parameters as a function of the position, also requires a special technology.
DE 10 2006 014 106 B3 discloses a device for measuring the density of a plasma, wherein a resonant frequency is determined in response to a high-frequency signal coupled into a plasma and used to calculate the plasma density. The device includes a plasma probe having a probe head in the form of a tri-axial ellipsoid that can be introduced into the plasma and means for coupling a high-frequency into the probe head via a shaft supporting the probe head. The probe head has a jacket and a probe core surrounded by the jacket, wherein the surface of the probe core has mutually insulated electrode regions of opposite polarity. The probe head has in particular the shape of a sphere, wherein the electrode regions have opposite polarity and are arranged parallel to the central transverse plane of the sphere. This probe design has a number of advantages arising from the mathematical concept of the multipole expansion.
The multipole expansion is a method which allows under certain conditions (separable coordinates) to explicitly resolve the mathematical relationships forming the basis for the equivalent circuit by using a formula. This results in an infinite sum representation, wherein however the weight of the higher-order multipole fields that correspond to the higher-order term of the sum decreases rapidly, so that the series can be truncated after only a few terms. Under certain circumstances, only the first sum term is significant, the so-called dipole component. When the ellipsoidal probe head and the wiring of the electrode regions are selected to be symmetrical with respect to a central transverse plane passing through the center, the zero-order sum term, i.e. the so-called monopole component, becomes zero. This leads to a simple and especially unambiguous evaluation rule, which allows the local plasma density to be determined with high accuracy.
However, it has also been shown that electrical coupling of the high-frequency signal via the probe shaft is demanding, since the electrodes have to be driven symmetrically with the high-frequency signal. The symmetrical control requires the feed line to also be electrically symmetrical, so as to eliminate phase shifts due to the routing of the conductors. This requires a relatively sophisticated wiring design for the preferably very small probes, especially for performing a spatially resolved measurement, which is only possible by moving the probe head.